Over the last decade, silicon-nitride Si3N4 balls have become an important component of advanced bearings used in a wide range of applications. The greatest commercial success for Si3N4 balls has been their use in hybrid bearings that combine the ceramic balls with steel races and that are known as silicon-nitride hybrid bearings. Compared to the steel balls, which the silicon-nitride balls replace, the ceramic balls are harder and less dense and offer higher compressive strength, better corrosion resistance, elevated operating temperature, and reduced lubrication requirements. These benefits make the hybrid bearings ideal for severe high-speed applications such as machine tool spindles, high-speed dental drills, vacuum turbomolecular pumps, and the liquid-oxygen turbomolecular pumps used in the space shuttle main engines. Large diameter ceramic balls recently became the leading technology for hip replacements.
For exemplar future use in the space industry, the hybrid bearings have been proposed for the improved momentum control wheels and flywheels for satellites. Importantly, the hybrid bearings have recently been used in roller blades, an application where the bearings represent a mass marketing opportunity for lightweight rugged bearings. The roller blade market, as well as other commercial applications, provides recreational users and athletes with cost-effective high technology long-lasting bearings with improved performance in high volumes that would lower the price of the hybrid bearings for all applications with increasing overall sales. For machine tool spindles, the market for hybrid bearings was at $35 million in 2000 and is projected to reach $150 million by 2005, and hence there is wide spread usage. The overall sales of hybrid bearings should reach several hundred million by 2010. Hence, there is a significant need for high-volume hybrid bearings subject to repeatable and accurate manufacturing requirements.
Ceramic balls have significant drawbacks and limitations. Like all ceramics, the silicon-nitride balls have a low tensile strength, which is a fundamental material property. Therefore, under applied tension, the balls are prone to crack either at a preexisting manufacturing flaw or at a flaw that develops during service and usage. A closely related fundamental material property is the fracture toughness, which indicates the susceptibility to fracture of the ceramic material. Low fracture toughness is the most important factor determining the ruggedness and usefulness of all ceramics in general as well as the silicon-nitride balls, in particular. Fortunately, highly engineered ceramics have been developed whose fracture toughness can be significantly increased through processing that controls microstructures. Precise manufacturing can control the size and number of preexisting flaws. To produce tougher ceramics is therefore a two-fold task. First, a microstructure is selected that is intrinsically tougher, which reduces the severity of any flaws. Second, the preexisting flaws are eliminated, which ameliorates the low fracture toughness.
In the specific case of silicon-nitride balls, manufacturing processing has been developed that provides, for example, a two-phase microstructure of alpha and beta silicon nitride where the minor second phase is a blocky shape in a matrix of the major phase. As a micromechanism, this microstructure promotes crack deflection and blunting, which raises the intrinsic fracture toughness. In addition, the ceramic balls are manufactured from a starting powder through hot-isostatic pressing that is followed by grinding and lapping to provide precise spherical shapes. Accurate control of the hot-isostatic pressing eliminates sintering voids and inclusions that are potential preexisting flaws leading to potential fracture and failure of the balls. Precise control of the grinding and lapping eliminates surface cracks. Inspection and nondestructive evaluations are also used to screen balls with preexisting flaws from usage especially in critical applications.
Another disadvantage for any brittle material is that any preexisting cracks are atomically sharp. Only a ductile material offers the crack tip blunting needed for a crack to depart from atomic sharpness. In the unloaded state, the opposite faces of an atomically sharp crack are close together and mate almost perfectly with a negligibly perceptible gap between the two faces.
Physically, the negligible gap minimizes the contrast of the crack exploited by conventional inspection methods. Hence, to conventional inspection methods, the atomically sharp crack appears to be healed and appears to be virgin uncracked material. That is, current inspection methods find it difficult to differentiate between cracked and uncracked material in a ceramic. The limit of resolution of current inspection methods is therefore raised when applied to a ceramic, and the smallest identifiable flaw is unfortunately quite high. Of course, the goal of any inspection method is to identify as small a flaw as possible.
The leading inspection technique used by the ball manufacturers is florescent penetrant dye. In this method, the ball is placed in a bath of liquid florescent dye. Any preexisting crack looks like a free surface to the liquid and therefore has a tendency to wick the dye into itself due to surface energy effects. The crack therefore is a potential reservoir for the dye. The ball is removed from the bath, wiped clean and viewed under black light. The preexisting crack will continuously ooze fluid, which will be visible under black light. The method relies upon the human eye to detect the contrast between florescent and unflorescent regions.
The florescent dye technique has physical limits on the smallest crack that can be resolved and detected. One natural limit is the acuity of human vision. Another limit for brittle ceramics is that an atomically sharp crack represents a very small reservoir and therefore a very low driving force to wick dye. Fundamentally, therefore, an atomically sharp crack provides very low contrast. For the florescent dye technique, the limit of resolution of crack depth exceeds 0.1 mm under ideal laboratory conditions. The limit probably exceeds 0.2 mm under practical factory conditions that must depend upon the attentiveness of an operator who becomes fatigued after many hours of inspection.
Other possible techniques rely upon reflective radiation, such as ultrasound, or transmissive radiation, such as x-ray. In the case of ultrasound, the preexisting flaw reflects an anomalous sound wave that is not reflected by the uncracked material. Physically, the reflection from the crack is the source of contrast that must be detected. Unfortunately, an atomically sharp flaw in a ceramic reflects a very small echo because the flaw appears to be almost perfectly closed. Only a relatively deep crack, on the order of 1.0 mm, becomes visible under the ultrasound method.
X-ray techniques rely upon differential absorption across the flaw, which is again very low for an atomically sharp flaw that appears to the X-ray to be closed. Another limitation is the sensitivity of the film or X-ray detector used to measure the differential absorption. The limit of resolution of a crack probably exceeds 1.0 mm for the X-ray method. As additional drawbacks, the ultrasound and X-ray techniques both require a well-trained operator and very expensive equipment. For these reasons, the X-ray and ultrasound techniques are currently not used by the ball manufacturers.
The hallmark of all current inspection methods is that a detector is used to identify a source of contrast possessed by a crack that is not possessed by uncracked material. A new replacement method is obviously needed to overcome the shortcomings of the current state-of-the-art inspection method, so that cracks much smaller than 0.1 mm can be reliably detected. If the new method can qualify the balls as defect-free to this finer limit of resolution, then the balls can be reliably used in new safety-critical applications such as turbine engines. A better inspection method offers two related benefits. First, the predicted lifetime of the ball becomes much longer. Second, the confidence in the lifetime is much greater.
The industrial inspection methods currently used have great difficulty resolving preexisting flaws that are atomically sharp because these flaws appear to be almost perfectly closed under the current methods. Disadvantageously, there is no current known test that can independently measure the fracture toughness of the ceramic balls. The existing inspection methods are disadvantageously not well suited to find cracks in brittle materials that are atomically sharp. Disadvantageously, the existing leading test method, florescent penetrant dye, can only detect surface cracks open to the environment and can not detect subsurface flaws, which are an important class of flaws leading to premature failure in service. All of the current industrial practices are disadvantageously highly labor intensive and rely upon the attentiveness and acuity of a human operator. These and other disadvantages are solved or reduced using the invention.